A subdomain is the section of the web address which is before a domain name and you've most likely seen a lot of subdomains while browsing world wide web. For instance, many websites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of employing a subdomain is that it can have an independent website and its own records, so you're able to even host it on another server. The practical use is that you can have a supplementary website, like an e-learning portal for students as well as the primary school site. If you are using subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it will be more secure to have the websites separate from one another.